1. Field of the Invention
The instant disclosure relates to an alternating current chopper circuit; in particular, to an alternating current chopper circuit with low noise.
2. Description of Related Art
A motor typically has a driving circuit to drive the motor and provide speed control. As shown in FIG. 1, a driving circuit of an induction motor 10 is illustrated. Of the shown driving circuit, the switching units Qa and Qb are turned on or off by the control signal VPWM1. The switching units Qc and Qd are turned on or off by the control signal VPWM2. The waveforms of the control signals VPWM2 and VPWM2 are shown in FIG. 1B.
By adjusting the duty cycle of the control signal VPWM1, the averaged power received by the motor 10 can be changed, thereby controlling the motor speed. Regardless of the load associated with the motor (e.g., resistive load or reactive load), the current flow must be continuous to maintain circuit stability. When the control signal VPWM1 is off, and no current-conducting path is provided to the motor 10, an instantaneous high voltage may be delivered to the motor components, which may damage the circuitry thereof. Therefore, controlled by the control signal VPWM2, the switching units Qc and Qd are used to provide a current-conducting path for the motor 10, when the switching units Qa and Qb are turned off.
When the control signal VPWM1 is at the high voltage level, and the control signal VPWM2 is at the low voltage level, the input voltage of the alternating current (AC) power Vin is supplied to the motor 10 via the switching units Qa and Qb. When the control signal VPWM1 is switched to the low voltage level, and the control signal VPWM2 is switched to the high voltage level, the motor 10 may utilize the switching units Qc and Qd to provide a path for passing the current, so as to maintain the continuous current flow through the motor 10 and prevent the components from being damaged.
To prevent short circuit by the AC power Vin, a dead time td exists in between the control signals VPWM1 and VPWM2, as shown in FIG. 1B. However, in the period of the dead time td, the current flow of the motor 10 is discontinued, with the surge current causing the damage to the switching units Qa, Qb, Qc, and Qd. Thereby, as shown in FIG. 1A, snubbering units comprised of the resistors Ra, Rb, Rc, and Rd and the capacitors Ca, Cb, Cc, and Cd are included. The resistors and capacitors are coupled to the respective switching units, to provide snubbering against the power surge due to discontinuous current at the switching units Qa, Qb, Qc, and Qd during the period of the dead time td.
Even with the snubbering unit, the voltages withstand by the switching units are inevitably unstable, which lead to malfunction and damage of the driver circuit. In addition, the RC (resistor-capacitor) snubbering units must be configured with different parameter values depending on the type of the motor 10 and different specifications of the switching units Qa, Qb, Qc, and Qd, complicating the design of the driver circuit.
Please refer to FIG. 2, which shows a driver circuit of an induction motor. When the switching unit Qe is turned on and the switching unit Qf is turned off, the AC power Vin, would magnetize the motor 10. When the switching unit Qe is turned off and the switching unit Qf is turned on, the motor 10 utilizes the current-conducting path provided by the switching unit Qf to maintain the continuous current flow. However, similar to the conventional motor shown in FIG. 1A, to prevent the occurrence of short circuit by the AC power Vin, a dead time period still exists in the conventional motor shown in FIG. 1B, and thus neither the switching units Qe nor Qf could be turned on immediately after the other has been turned off. Thereby, during the dead time period, the current flow of the motor still would be discontinuous which may damage the circuitry.